Space Forum / Space Flight / September 2005

[...]

I Am Not A Rocket Scientist, but my gut feeling is that as the outside
pressure drops, you'd feel the effects of not having a nozzle more and more
- --- the difference between the desired exit diameter to get the optiumum
expansion ratio and the *actual* exit diameter would increase hugely. But I
don't know enough about the figures to tell for sure. Carmack is, of
course, doing all his testing at sea level.

But I think he's right in that not using nozzles allows him a much greater
turnover in engine prototypes. I'm interested to see how it goes.

- --
+- David Given --McQ-+

+- www.cowlark.com --+

"manofsan@yahoo.com" <manofsan@gmail.com> :

And it is a good thing that he did too!

I don't know myself, but I know years ago when I thought of looking at them
that the articles I read on rockets all said nozzleless design took a big hit
in the ISP.  I even thought of it again about two years ago and again dropped
the idea thinking the loss in ISP was going to be too great.

John did the right thing.  Instead of assuming the books/articles were all
right he did some tests to get the facts himself.  Guess what?  Conventional
wisdom is not always right.  A good clue to this is if you do a Google
search, STINET or NASA TECHNICAL REPORTS SERVER the low count of hits
indicate a lack of published research.

Also he wrote the following:
"I was recently looking at the table in Sutton regarding losses due to small
chamber to throat contraction ratios, and they weren't as significant as I
had remembered them.  A chamber with no contraction ratio at all will lose
20% of its thrust due to pressure losses from accelerating gasses in the
straight section, but the Isp loss is only 1.5%"

This also suggests the other problem, Bad Memories.  Notice the Sutton book
mentions there is a large lost in thrust not ISP?  Overtime people memories
sometimes get mixed up and they start remembering 20% lost in ISP not thrust,
they then write this in thier articles or teach thier students the same and
as most research is in getting the *most* thrust and/or ISP out of an engine
no-one does backup experiments to see if the figures match.

Reducing dry mass in a rocket is always a good thing if you don't increase
the risk of failure modes.  Infact this rocket design seems to the simpler
still than previous designs and can be expected to have fewer failure modes
which is all to the good.  On the other hand they just saw what I think is a
new failure mode to them (buckling) that as far as I can remember never
happened with any of thier other designs.
 

Simple to build, parts (mostly pipes) are off the shelf items, parts to scale
up in size are easy to get, cheap/fast to make means destructive testing does
not cost too much to do - damn it I am going out for parts to build one now
too!

             Earl Colby Pottinger